Biology Past Paper Questions Answers PDF

Review Questions Exam #4, Biology 112-17, -18, -19 1) What is the purpose of mitosis? - MITOSIS IS THE PROCESS IN WHICH NEW SOMATIC (BODY) CELLS ARE CREATED -TYPE OF CELL DIVISON - CREATED SOMATIC CELLS THAT ARE DIPOLID (2N) - GOOD FOR REPAIR OF DAMAGE...

Review Questions Exam #4, Biology 112-17, -18, -19 1) What is the purpose of mitosis? - MITOSIS IS THE PROCESS IN WHICH NEW SOMATIC (BODY) CELLS ARE CREATED -TYPE OF CELL DIVISON - CREATED SOMATIC CELLS THAT ARE DIPOLID (2N) - GOOD FOR REPAIR OF DAMAGE DONE TO THE BODY - MAKES IDENTICAL CELLS 2) What is a gamete and somatic cell? What is a zygote? - SOMATIC CELLS ARE BODY CELLS (SKIN CELLS, MUSCLE CELLS) - GAMETE CELLS ARE REPRODUCTIVE CELLS AND ARE HAPLOID (N) - ZYGOTE IS A FERTALIZED EGG (EGG AND SPERM JOIN TOGETHER) 3) What is the “N” value of a cell? What do haploid, diploid, and tertraploid mean? - “N” VALUE OF A CELL IS WHAT HELPS INDICATE THE NUMBER OF SETS OF CHROMOSOMES (N, 2N, 4N) - HAPOLID IS A SINGLE SET OF CHROMOSOMES, DIPOLID IS A DOUBLE SET, TETRAPLOID IS 4 SETS OF CHROMOSOMES 4) What are the phases of mitosis? What happens in each phase? PHASES OF MITOSIS: 1. INTERPAHSE: THIS IS WHERE THE CELL CYCLE TAKES PLACE (G1 (GROWTH), S(SYNTHESIS), G2(PREPERATION FOR MITOSIS) 2. PROPAHSE: CHROMOSOMES ARE CONDESNING (THICKENING AND VISIBLE) 3. METAPHASE: CHROMOSOMES LINE UP IN THE MIDDLE OF THE CELL (METAPHASE PLATE) 4. ANAPHASE: SISTER CHROMOTIDS SPLIT AND SPINDLE FIBERS MOVE THEM TO OPPOSITE SIDE OF CELL 5. TELOPHASE: CHROMOSOMES ARE AT EACH END SIDE OF CELL & NUCLEI ARE FORMING TO MAKE NEW CELLS 5) Why is prophase divided in early and late prophase (i.e. pro-metaphase)? - BECAUSE SIGNIFICANT CHANGES HAPPEN DURING THIS PHASE, SEPARATING IT INTO 2 PHASES HELPS CLARIFY THE EVENTS EARLY PROPHASE: - CHROMOSOMES CONDENSE AND BECOME VISIBLE AND THICKER - CONTOSOMES START MOVING TO OPPOSIDE SIDES PREAPRING TO ORGANIZE SPIDLES - MITOIC SPINDLE FIBERS START TO FORM - NUCLEAR EVENELOPE IS STILL INTACT LATE PROPHASE: - NUCLEAR ENVELOP BREAKS DOWN - SPINDLE FIBERS ATTACH TO KINETOCHORES - CHROMOSOMES START MOVING TOAWRDS THE CENTER OF THE CELL 6) What is a centriole? Spindle fibers? The metaphase plate? Sister chromatids? Homologous chromosomes? Centromere? Kinetochore? - CENTRIOLW: SMALL CYLINDRICAL STRUCTURE IN ANIMAL CELLS TRHAT ORGANIZES THE SPINDLE FIBERS IN CELL DIVISION - SPINDLE FIBERS: HELP MOVE THE CHROMOSOMES TO THE END SIDES OF THE CELL - METAPHASE PLATE: IMAGINARY PLANE WHWRE CHROMOSOSMES ALIGN DURING METAPHASE  Sister chromatids: Two identical copies of a chromosome after DNA replication.  Homologous chromosomes: Chromosomes that are similar in shape and size, one from each parent.  Centromere: Region where two sister chromatids are joined.  Kinetochore: Protein structure at the centromere where spindle fibers attach. 7) How do we tell chromosomes apart? What criteria? What is a karyotype? -A karyotype is an image showing the number and appearance of chromosomes. Chromosomes are identified by their size, shape, and banding pattern. 8) What is nondisjunction? - Nondisjunction occurs when chromosomes fail to separate properly during meiosis or mitosis, leading to cells with an abnormal number of chromosomes. 9) What do we mean when we say chromosomes are “fully condensed”? Chromosomes are considered fully condensed when they are tightly coiled and visible under a microscope, typically during metaphase. 10) What is cytokinesis and the cleavage furrow?  Cytokinesis: The final step of cell division, where the cytoplasm divides and two daughter cells are formed.  Cleavage furrow: The indentation that forms during cytokinesis in animal cells as the cell divides. 11) What is cancer? What are oncogenes? What are tumor suppressors? How can mistakes in the cell cycle lead to cancer?  Cancer is uncontrolled cell division, often due to mutations in genes regulating the cell cycle.  Oncogenes: Genes that can cause cancer when mutated.  Tumor suppressors: Genes that prevent cancer; mutations can lead to cancer if they are inactivated. 12) What is retinoblastoma? What causes it? Retinoblastoma is a type of eye cancer caused by mutations in the RB1 gene, a tumor suppressor gene. 13) What is a pulse labeling experiment? What was it used to find out in our lecture discussion? A pulse labeling experiment tracks the incorporation of labeled molecules (like nucleotides) into cells to study DNA replication and other processes. 14) How does bacterial cell division work? Bacteria divide by binary fission, where the cell replicates its DNA and then splits into two identical daughter cells. 15) How does plant cell division differ from animal division?  Plant cells form a cell plate during cytokinesis, while animal cells form a cleavage furrow.  Both processes involve similar phases in mitosis. 16) Where does genetic variation come from in meiosis? Genetic variation in meiosis comes from crossing over and independent assortment, which shuffle genetic material between chromosomes. 17) What is synapsis and when does it take place? Synapsis is when homologous chromosomes pair up during prophase I of meiosis to form tetrads. 18) What is crossover (homologous recombination) and when does it take place? Crossover occurs during prophase I of meiosis, where homologous chromosomes exchange genetic material, increasing genetic diversity. 19) What are the phases of meiosis? What are there two sets of divisions? What happens in each division?  Meiosis I: Homologous chromosomes separate.  Meiosis II: Sister chromatids separate, similar to mitosis. 20) When do the chromosomes separate in meiosis? In mitosis?  In meiosis, chromosomes separate during anaphase I and anaphase II.  In mitosis, sister chromatids separate during anaphase. 21) What is aneuploidy? What is polyploidy?  Aneuploidy: Abnormal number of chromosomes (e.g., trisomy).  Polyploidy: More than two sets of chromosomes (e.g., triploid, tetraploid). 22) What is Down syndrome and its cause? Down syndrome is caused by trisomy 21 (three copies of chromosome 21). 23) What is bacterial “sex”? Bacteria exchange genetic material through conjugation, where DNA is transferred from one cell to another. 24) How are mitosis and meiosis the same? How do they differ?  Mitosis produces identical diploid cells, used for growth and repair.  Meiosis produces haploid gametes, used in sexual reproduction. 25) What were the historical models for genetics before Mendel? Before Mendel, the prevailing theories were blending inheritance and pangenesis (the idea that traits were passed through particles called “gemmules”). 26) What is a pure line? A hybrid? Phenotype? Genotype?  Pure line: Organisms that produce offspring identical to themselves.  Hybrid: Offspring resulting from the cross of two different pure lines.  Phenotype: The physical expression of traits.  Genotype: The genetic makeup of an organism. 27) What are dominant traits? Recessive traits? Alelles?  Dominant traits: Traits that appear even with one allele (e.g., brown eyes).  Recessive traits: Traits that appear only if both alleles are recessive (e.g., blue eyes).  Alleles: Different versions of a gene. 28) What is a perfect flower? How did Mendel cross pea plants? Mendel studied pea plants and used crossbreeding to discover patterns of inheritance. He observed traits like flower color and seed shape. 29) What is a Punnett square and how is it used? A Punnett square is a tool used to predict the probability of offspring genotypes and phenotypes based on parental alleles. 30) What do the letters around a Punnett square represent? The letters represent the alleles of the parents, with uppercase letters for dominant alleles and lowercase letters for recessive alleles. 31) If you cross two monohybrid heterozygotes, what ratio will the phenotypes be of the offspring? When crossing two heterozygotes, the phenotypic ratio of offspring will typically be 3:1 (dominant: recessive). 32) What does wild-type mean? Wild-type refers to the most common allele or phenotype in a population. 33) What is reciprocal cross? A reciprocal cross involves switching the traits of the parents (e.g., crossing a homozygous dominant female with a homozygous recessive male and vice versa). 34) What is a dihybrid cross? If you cross two dihybrid heterozygotes, what ratio will the phenotypes be of the offspring? In a dihybrid cross (two traits), crossing two heterozygotes results in a 9:3:3:1 phenotypic ratio. 35) What is Mendel’s law of Independent assortment? This law states that alleles for different traits segregate independently during meiosis. 36) What is non-Mendelian genetics? Non-Mendelian genetics refers to patterns of inheritance that don’t follow Mendel’s laws, such as incomplete dominance or codominance. 37) What is a Sex-linked trait? Why are X-linked recessive phenotypes so rare in females? Sex-linked traits are carried on the sex chromosomes (usually the X). X-linked recessive traits are more common in males because they have only one X chromosome. 38) What is incomplete dominance? Multigenetic inheritance? What is a sex-limited trait? What is a hemizygosity?  Incomplete dominance: A mix of two alleles (e.g., red and white flowers producing pink flowers).  Multigenetic inheritance: Traits controlled by more than one gene.  Sex-limited traits: Traits that only appear in one sex.  Hemizygosity: Having only one allele for a gene, often in males for X-linked genes. 39) How do chickens, grasshoppers, and blowflies determine gender? Are male or female chickens hemizygous?  Chickens: ZW (female) and ZZ (male).  Grasshoppers: XX (female) and XO (male).  Blowflies: XX (female) and XY (male). 40) What is Klinefelters, Turners, and Jacob’s Syndromes? What causes them and what is the genotype of each? What are autosomes?  Klinefelter Syndrome: XXY, affects males with additional X chromosome.  Turner Syndrome: X0, affects females with only one X chromosome.  Jacob’s Syndrome: XYY, affects males with an extra Y chromosome. 41) What is X-inactivation? Why is this important is human females that have a trisomy for the X chromosome? Why are survivalable autosomal trisomys so rare in humans? X-inactivation in females shuts down one X chromosome to equalize gene expression between males and females. 42) What is an inborn error of metabolism? What is alkaptonuria? What is the one-gene-one enzyme hypothesis?  Inborn errors are genetic disorders that disrupt normal metabolic pathways.  Alkaptonuria is a metabolic disorder where a substance builds up in the body. 43) What is Occam’s razor? How was the genetic code determined? Occam’s Razor suggests that the simplest explanation is usually correct. The genetic code was determined by experiments that identified the relationship between codons and amino acids. 44) What are the characteristics of the genetic code? What is degeneracy? What is the start codon and AA it codes for? What are the three stop codons? How many codons are there in total? Why is the code universal?  Degeneracy: Multiple codons can code for the same amino acid.  The start codon is AUG, which codes for methionine.  Stop codons: UAA, UAG, UGA. 45) Know the following types of mutations: substitution, insertion, deletion, frame shift, silent, missense, nonsense. What are the consequences of each?  Substitution: One base is replaced by another.  Insertion: An extra base is added.  Deletion: A base is removed.  Frame-shift: Insertion or deletion shifts the reading frame.  Silent: No change in the protein.  Missense: A change in one amino acid.  Nonsense: A premature stop codon. 46) What causes sickle cell anemia? Sickle cell anemia is caused by a mutation in the hemoglobin gene (specifically in the beta-globin chain of hemoglobin). The mutation causes the amino acid glutamic acid to be replaced with valine at position 6 of the hemoglobin protein. This causes the hemoglobin to stick together under low oxygen conditions, resulting in sickle-shaped red blood cells, which can block blood flow and cause pain and organ damage. 47) What is the Central Dogma of biology? The Central Dogma of biology describes the flow of genetic information. It states that:   DNA is transcribed into mRNA (messenger RNA).  mRNA is translated into proteins at the ribosome. This concept highlights how genetic information in DNA is used to create proteins, the functional molecules in cells. 48) Recognize the four DNA bases and its basic structure—and the difference between DNA and RNA structure. What is an antiparallel structure?  DNA Bases:  Adenine (A) pairs with Thymine (T).  Cytosine (C) pairs with Guanine (G).  DNA Structure:  DNA is a double helix made of two strands, each containing a sugar-phosphate backbone and nitrogenous bases.  RNA has ribose sugar instead of deoxyribose, and uracil (U) replaces thymine (T).  Antiparallel Structure: In a DNA molecule, the two strands run in opposite directions. One strand runs 5’ to 3’, and the other runs 3’ to 5’. This is called "antiparallel" orientation. 49) What are Purines? Pyrimidines? Sugar-phosphate backbone? dNTPs? NTPs? Phosphodiester bonds?  Purines: Adenine (A) and Guanine (G) – have two rings in their structure.  Pyrimidines: Cytosine (C), Thymine (T), and Uracil (U) – have one ring.  Sugar-phosphate backbone: This is the structural framework of nucleic acids (DNA/RNA), consisting of alternating sugar (deoxyribose or ribose) and phosphate groups.  dNTPs (Deoxyribonucleotide triphosphates): The building blocks of DNA, consisting of a deoxyribose sugar, a phosphate group, and a nitrogenous base (A, T, C, G).  NTPs (Nucleoside triphosphates): The building blocks of RNA, consisting of ribose sugar, a phosphate group, and a nitrogenous base (A, U, C, G).  Phosphodiester bonds: These are the bonds that link nucleotides together in the sugar-phosphate backbone of DNA or RNA. 50) What are conservative and semi-conservative replication? What is a replication fork?  Conservative replication: The original DNA molecule stays intact, and a completely new copy is made.  Semi-conservative replication: Each new DNA molecule consists of one old strand and one newly synthesized strand. This is the method of replication in cells.  Replication fork: It is the Y-shaped region where the DNA is split into two strands for replication. It's where the DNA is actively being unwound and copied. 51) Number of hydrogen bonds between GC and AT?  GC pairs: 3 hydrogen bonds.  AT pairs: 2 hydrogen bonds. 52) The DNA synthesis reaction? Condensation reaction? What do in vivo and in vitro mean? What is Bidirectional replication?  DNA Synthesis Reaction: The process by which DNA is replicated, where nucleotides are added to the growing strand by DNA polymerase, and phosphodiester bonds are formed.  Condensation Reaction: A reaction where two molecules combine, and a water molecule is released. In DNA synthesis, nucleotides are added, and water is released.   In vivo: Refers to processes that occur inside a living organism.  In vitro: Refers to processes that occur outside a living organism, such as in a test tube.  Bidirectional replication: DNA replication that occurs in both directions from the origin of replication, forming two replication forks. 54) What are the limitations on DNA polymerase? What can it do and what can’t it do? DNA polymerase:  Can add nucleotides to the growing DNA strand.  Can proofread and correct mistakes during replication.  Cannot start a new DNA strand; it requires a primer to begin.  Cannot synthesize in the 3’ to 5’ direction; it only works in the 5’ to 3’ direction. 55) What DNA direction does synthesis take place? What chemical structures must be in place for synthesis to extend?  Synthesis direction: DNA synthesis occurs in the 5' to 3' direction.  Chemical structures needed:  A primer is required to start DNA synthesis.  The necessary nucleotides (dNTPs) are added to extend the chain.  DNA polymerase catalyzes the process. 56) What is a primer and why is it important? What primers are generated in DNA synthesis in vivo? What is a leading strand? A lagging strand? An Okazaki fragment?  Primer: A short strand of RNA or DNA that provides a starting point for DNA polymerase to begin replication.  Primers in vivo: In cells, RNA primers are synthesized by primase.  Leading strand: The strand of DNA that is synthesized continuously in the 5' to 3' direction.  Lagging strand: The strand of DNA that is synthesized in fragments, in the opposite direction of the replication fork.  Okazaki fragments: Short segments of DNA synthesized on the lagging strand. 58) In prokaryotes, what are the functions of DNA polymerase I., DNA polymerase III, ligase, topoisomerase, helicase, primase, and single strand binding protein?  DNA polymerase I: Removes RNA primers and replaces them with DNA.  DNA polymerase III: Synthesizes new DNA strands by adding nucleotides.  Ligase: Joins Okazaki fragments together by forming phosphodiester bonds.  Topoisomerase: Relieves tension in the DNA by making temporary cuts.  Helicase: Unwinds the DNA double helix ahead of the replication fork.  Primase: Synthesizes RNA primers to start replication.  Single-strand binding protein: Binds to single-stranded DNA to prevent it from re-annealing during replication. 59) What causes mutations in DNA? What are gene duplications? Chromosomal inversions? What is mismatch repair? How can cells tell which DNA strand should be repaired? What does DNA methylation have to do with this?  Causes of mutations: Mutations can be caused by environmental factors like UV radiation, chemical exposure, or errors during DNA replication.  Gene duplications: When a segment of DNA is copied more than once.  Chromosomal inversions: When a portion of a chromosome is reversed end-to-end.  Mismatch repair: A repair system that corrects errors in DNA that were missed during replication.  Strand identification: Cells recognize the newly synthesized strand (which is more likely to contain errors) because it has a different pattern of methylation compared to the template strand.  DNA methylation: Addition of methyl groups to DNA that can affect gene expression and repair processes. 61) What is the function of RNA polymerase? mRNA? tRNA, rRNA? Does RNA pol need a primer to work? Does it move in the same direction as DNA pol? What is a “processive” enzyme?  RNA polymerase: Synthesizes RNA from a DNA template.  mRNA (messenger RNA): Carries genetic information from DNA to the ribosome for protein synthesis.  tRNA (transfer RNA): Brings amino acids to the ribosome during translation.  rRNA (ribosomal RNA): Forms part of the ribosome and is involved in protein synthesis.  Primer requirement: RNA polymerase does not require a primer to begin transcription.  Direction: RNA polymerase moves in the 5' to 3' direction, just like DNA polymerase.  Processive enzyme: An enzyme that can catalyze many reactions without releasing its substrate. 62) What is the function of a promoter? What is the structure of a bacterial promoter? What is a consensus sequence? What is sequence conservation? What does the sigma subunit do?  Promoter: A region of DNA that signals where RNA polymerase should start transcription.  Bacterial promoter: Contains two conserved sequences, the -35 and -10 regions, which are recognized by RNA polymerase.  Consensus sequence: A sequence that represents the most common nucleotides found at specific positions across different genes.  Sequence conservation: Refers to the preservation of the sequence of nucleotides across different species or individuals.  Sigma subunit: A component of bacterial RNA polymerase that helps it recognize and bind to the promoter. 63) What is an operon? What is a polycistron? When is bacterial mRNA mature? What is the lactose operon? When is eukaryotic mRNA mature? What needs to happen for it to get mature? What is a 5’CAP? What is a 3’ Poly-A tail? What is an intron and exon?  Operon: A group of genes in bacteria that are transcribed together under the control of one promoter.  Polycistron: A single mRNA molecule that codes for multiple proteins.  Bacterial mRNA maturity: Bacterial mRNA is mature as soon as it is transcribed, as bacteria do not have introns.  Lactose operon: A group of genes in bacteria that are involved in the metabolism of lactose. It is regulated by the presence or absence of lactose.  Eukaryotic mRNA maturity: Eukaryotic mRNA needs to be processed (e.g., 5' cap addition, poly-A tail addition, and splicing) before it can exit the nucleus.  5' CAP: A modified guanine nucleotide added to the 5' end of mRNA to protect it from degradation and help with translation initiation.  3' Poly-A tail: A chain of adenine nucleotides added to the 3' end of mRNA, which helps with mRNA stability and export.  Introns: Non-coding regions in eukaryotic genes.  Exons: Coding regions in eukaryotic genes. 64) Where do transcription and translation occur in eucaryotes? The same place? What are the physical differences in the mRNA ade by a eukaryote and a prokaryote? What are the differences in transcription between eukaryotes and prokaryotes?  Transcription and translation locations:  In eukaryotes, transcription occurs in the nucleus, and translation occurs in the cytoplasm.  In prokaryotes, both transcription and translation occur in the cytoplasm since prokaryotes lack a nucleus.  mRNA differences:   Eukaryotic mRNA: Has introns removed (spliced), 5' cap, and 3' poly-A tail.  Prokaryotic mRNA: Is often polycistronic and does not undergo extensive processing.  Transcription differences:  Eukaryotes: Have more complex transcription machinery, and transcription is separated from translation by the nuclear membrane.  Prokaryotes: Transcription and translation occur simultaneously in the cytoplasm. 65) What are the minimal requirements for translation to take place? What is the basic structure of a tRNA and what are a codon and anticodon? What is the binding site for AA’s on a tRNA?  Ribosome: A complex molecular machine that facilitates the translation of mRNA into proteins. It is made up of rRNA and proteins.  Translation: The process by which ribosomes read mRNA sequences and synthesize proteins.  Initiation: The small ribosomal subunit binds to the mRNA, and the initiator tRNA (carrying methionine in eukaryotes) binds to the start codon. The large ribosomal subunit then associates with the small subunit to form a functional ribosome.  Elongation: The ribosome moves along the mRNA, and tRNAs bring amino acids to the ribosome, where the amino acids are linked by peptide bonds.  Termination: When a stop codon is encountered, the ribosome releases the newly synthesized protein, and the ribosome dissociates from the mRNA.  Ribosome sites:  A site (Aminoacyl site): Where the tRNA carrying the next amino acid enters.  P site (Peptidyl site): Where the tRNA holding the growing polypeptide chain resides.  E site (Exit site): Where the tRNA, after donating its amino acid, exits the ribosome. 66) What are the major steps of translation? What is the initiation step? What is the Shine-Dalgarno sequence and its function? What is a formyl- methionine vs. a plain methionine? What is elongation? What is a ribozyme? What is an aminoacyl tRNA? What are the functions of the A, P, and E sites? What causes termination of translation?  Major steps of translation: 1. Initiation: The small ribosomal subunit binds to the mRNA at the start codon. The first tRNA (carrying formyl-methionine in prokaryotes) binds to the start codon, and the large ribosomal subunit joins to form a functional ribosome. 2. Elongation: The ribosome moves along the mRNA, and tRNAs bring amino acids corresponding to the codons. Peptide bonds are formed between the amino acids to grow the polypeptide chain. 3. Termination: When a stop codon is encountered, release factors bind to the ribosome, releasing the newly synthesized protein and dissociating the ribosome.  Shine-Dalgarno sequence: A short sequence in bacterial mRNA that helps the ribosome bind. It is recognized by the 16S rRNA of the ribosome, ensuring the ribosome starts translating at the correct place.  Formyl-methionine (fMet) vs. Methionine (Met):  fMet is the first amino acid in bacterial translation, specifically used in initiation.  Met is the first amino acid in eukaryotic translation.  Elongation: During elongation, tRNAs bring amino acids to the ribosome, matching their anticodons with the mRNA codons. The ribosome moves one codon at a time, linking the amino acids together to form the protein.  Ribozyme: A type of RNA that acts as an enzyme, catalyzing chemical reactions. The ribosome itself is a ribozyme, catalyzing peptide bond formation.  Aminoacyl tRNA: A tRNA molecule that is charged with its corresponding amino acid. It delivers the correct amino acid to the ribosome during translation.  A, P, and E sites on the ribosome:  A site (Aminoacyl site): The site where the incoming aminoacyl-tRNA binds.  P site (Peptidyl site): The site where the tRNA holding the growing polypeptide is located.  E site (Exit site): The site where the tRNA exits the ribosome after donating its amino acid.  Termination of translation: Termination occurs when a stop codon is encountered. Release factors bind to the stop codon, causing the ribosome to release the polypeptide and dissociate from the mRNA. 67) What’s the difference between constitutive and regulated expression? At what levels can gene expression be regulated? What is negative and positive gene regulation? What is an inducer? What is a repressor? What does beta-galactosidase do?  Constitutive expression: Some genes are always "on" and make proteins all the time.  Regulated expression: Some genes are turned "on" or "off" depending on needs.  Levels of gene regulation:  Transcription level: Control whether a gene is copied into mRNA.  Post-transcriptional level: Control what happens to the mRNA after it's made.  Translational level: Control whether the mRNA is used to make protein.  Post-translational level: Control what happens to the protein after it's made.  Negative regulation: A repressor stops gene expression by blocking the gene.  Positive regulation: An activator helps turn on gene expression.  Inducer: A molecule that turns a gene "on" by inactivating a repressor.  Repressor: A protein that stops a gene from being expressed by blocking it.  Beta-galactosidase: An enzyme that helps break down lactose in bacteria. 68) How does the Lac operon function? What is an operator sequence? What must be present as an inducer to get the Lac Operon to express?  Lac operon function: This is a set of genes in bacteria that control the breakdown of lactose.   Without lactose: A repressor stops the genes from working.  With lactose: Lactose binds to the repressor, turning it off, and allowing the genes to work to break down lactose.  Operator sequence: A spot on the DNA where the repressor binds to stop gene expression.  Inducer: Lactose is the inducer that turns the Lac operon "on" by inactivating the repressor. 69) What is a DNA binding protein? What do these have to do with Lac Operon and its repressor protein? What is a helix-loop-helix motif?  DNA binding protein: These are proteins that bind to specific parts of DNA to control gene expression.  Lac operon and repressor: The Lac repressor is a DNA binding protein that stops the genes from being expressed by binding to the operator. When lactose is present, the repressor is turned off and the genes are allowed to work.  Helix-loop-helix motif: This is a shape in some DNA binding proteins that helps them attach to DNA to control genes.